The present invention relates to a power amplifier which amplifies UHF, micro wave and millimeter wave signals with a wide dynamic range.
Recently, there is an increasing demand for power amplifiers as one of the key device of pocket telephone in a mobile communication system. A high gain, high output power, and high efficiency are required for such a power amplifier.
FIG. 12 is a block diagram of one of the examples of conventional power amplifiers. As shown in FIG. 12, FET 403 and FET 408 are connected in cascade with their sources grounded. The gates of FETs 403 and 408 are connected both with input matching circuits 402 and 407 and with gate bias terminals 404 and 409, respectively, while the drains of FETs 403 and 408 are connected both with draun bias terminals 405 and 410 and with output matching circuits 406 and 411, respectively. Further, the drain of FET 403 is connected with the gate of FET 408 through output matching circuit 406 and input matching circuit 407.
Further, in order to improve a spectral efficiency of the pocket telephones, code division multiple access (CDMA) using spread spectrum is being put into practical use. It is required in the CDMA that transmission output power be controlled within 80 dB to cope with the near-far problem, and that an envelope be detected precisely. Accordingly, the linearity of the amplifier for CDMA should be more strictly guaranteed than the conventional amplifiers.
FIG. 13 is a block diagram of one of the conventional transmission output power controller. The conventional power controller as shown in FIG. 13, which controls the power inputted into power amplifier 416 by using variable gain amplifier 415, comprises radio frequency signal generator 413, modulator 414 for modulating the output from radio frequency signal generator 413, power amplifier 416 for amplifying the output from modulator 414, variable gain amplifier 415 for controlling power amplifier 416, antenna duplexer, and antenna 418.
As mentioned above, the amplifier for CDMA should be a low distortion, high power, and linear amplifier.
However, the conventional amplifiers have a disadvantage that the electric power is consumed vainly, because the power efficiency by the combination of the variable gain amplifier and the power amplifier is much smaller for the minimum power input than for the maximum power input.
Therefore, an object of the present invention is to provide a power amplifier with high power efficiency, low distortion, and variable gain in wide dynamic range.
To solve the above mentioned problems, the power amplifier of the present invention comprises an input stage including one or more FETs with grounded sources, and an output stage including one or more FETs with grounded gates. Further, a plurality of bias terminals are connected with the sources and drains of the transistors in the output stage. When the output power is made smaller than a prescribed threshold, the power inputted into the output stage is allowed to go through without being amplified from the output terminal of the output stage, by applying a control voltage to the bias terminals.
Another power amplifier of the present invention comprises an input stage including one or more bipolar transistors with grounded emitters, and an output stage including one or more bipolar transistors with grounded bases. Further, a plurality of bias terminals are connected with the sources and drains of the transistors in the output stage. When the output power is smaller than a prescribed threshold, the power inputted into the output stage is allowed to go through without being amplified from the output terminal of the output stage, by applying a control voltage to the bias terminals.
Still another power amplifier of the present invention comprises a plurality of transistors, at least two of which are FETs with grounded gates. Further, a plurality of bias terminals are connected with the sources and drains of the FETs with grounded gates. When the output power is smaller than a prescribed threshold, the power inputted into at least one of the FETs with grounded gates is allowed to go through without being amplified, by applying a control voltage to the bias terminal.
Other power amplifier of the present invention comprises a plurality of transistors, at least two of which are bipolar transistors with grounded bases. Further, a plurality of bias terminals are connected with the emitters and collectors of the bipolar transistors with grounded bases. When the output power is smaller than a prescribed threshold, the power inputted into at least one of the bipolar transistors with grounded bases is allowed to go through without being amplified, by applying a control voltage to the bias terminal.
Further, one or more FETs with grounded gates in the input stage as mentioned-above may be connected with one or more first input-output matching circuits which allows the linear gain of the input stage to be greater than that of the output stage, while one or more FETs with grounded gates in the output stage as mentioned-above may be connected with one or more second input-output matching circuits which allows the output power from the output stage to be greater than that of the input stage.
Further, one or more bipolar transistors with grounded bases in the input stage as mentioned-above may be connected with one or more first input-output matching circuits which allows the linear gain of the input stage to be greater than that of the output stage, while one or more bipolar transistors with grounded bases in the output stage as mentioned-above may be connected with one or more second input-output matching circuits which allows the output power from the output stage to be greater than that of the input stage.
In short, the input power into at least one of the transistors of the power amplifier of the present invention is passed through without amplification, when the output power of the power amplifier of the present invention is made smaller than a prescribed threshold. Therefore, the power efficiency becomes high, even when the transmission power is small.